Radar, since its rapid development during World War II, has become a primary sensor for both military and nonmilitary operations. Continuing developments have greatly improved the operational capabilities of radar systems; however, parallel increases in operational requirements have increased technological deficiencies. A relatively large and specialized vocabulary has evolved that relates to radar problems and operational requirements: e.g., clutter, electronic counter measures (ECM), jamming, chaff, decoys, anti-radiation missiles (ARM), low probability of intercept (LPI), identification of friend or foe (IFF), electromagnetic interference or control (EMI/EMC), high density targets, etc. The solutions to all the above problems relate to improving resolution in the space, time and frequency domains, and increasing the RF to information bandwidth ratio. The problems and solutions are interrelated and are often at cross purposes; e.g., increased resolution often adversely affects data rates. The development of conventional radar has been directed towards higher average power, pulse repetition rates, duty cycles, pulse compression ratios and improved Doppler frequency resolution. Unfortunately, processing losses associated with signal eclipsing and functional time sharing along with system complexity have also increased. A new generic type of radar design architecture is required. CFMR (coherent frequency multiplexed radar) offers solutions to many of these categorical problems.
Radar operates in the time, space and frequency domains. Transmitter and receiver signal isolation can be achieved in any of these domains. In pulse radar the transmitting and receiving periods are separated in the time domain. Signal separation in CW radar systems, except for some special purpose low power application, is achieved in the space domain (separate antennas). In CFMR the transmitted wave is continuous, but is comprised of a series of contiguous pulses transmitted at different frequencies in such a manner that they can be separated, from each other and from the signal being transmitted, by frequency multiplex techniques. Furthermore, a coherent relationship between the voltage vector of each frequency segment transmitted provides for simultaneous measurement of target range and velocity with optimum processing gain maintained.
CFMR is a continuous wave system and must therefore be able to receive while transmit. It can, in part, utilize the same implementation as conventional continuous wave radar; primarily, separate transmitting and receiving antennas, duplexors (circulators with canceller and Doppler frequency shift. CFMR, by virtue of its coherent frequency segmented signal format, has the further advantage of separating the signals through frequency multiplexing techniques. Foregoing the option of dual antennas for transmit and receive functions, the duplexor in CFMR operation reduces the coupling between the transmitter's output and the receiver's input (radio frequency preamplifier and/or first mixer) such that the residual transmitted power, as observed at the input of the receiver, is insufficient to cause burnout, excessive overloading to introduce undesirable non-linearities or degrade the receiver's noise figure. In CFMR the frequency separation of individual segments making up the signal format is accomplished at IF and/or baseband frequencies through frequency multiplexing by means of analog filters and/or digital filtering techniques utilizing appropriate time sampling in the radio frequency detection process. Doppler extraction is accomplished in much the same manner as with conventional Doppler radar, i.e., further dividing coherent baseband frequencies into Doppler frequency bands. Signal processing is conveniently implemented through digital computer techniques.
A comparison of the pertinent operational features of the radar technologies are tabulated below:
__________________________________________________________________________ Resolution Ambiguities Radar Range Velocity Range Velocity Processing Type (Time) (Doppler Freq) (Time) (Doppler Freq) Gain __________________________________________________________________________ Pulse or Pulse Doppler ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## Pulse Compression ##STR6## -- ##STR7## -- ##STR8## Doppler ##STR9## ##STR10## -- none .alpha. T.sub.i' CFMR ##STR11## ##STR12## ##STR13## ##STR14## .alpha. T.sub.i' __________________________________________________________________________ where c = speed of light f.sub.c = carrier frequency PRF = Pulse Reception frequency .alpha. = PROPORTIONAL TO T.sub.i = integration period .tau. = pulse or frequency segment width .tau..sub.e = 1/total bandwidth of transmitted signal
Note brackets 1 and 2. CFMR has the range resolution of pulse compressed radar and the Doppler resolution of CW radar. Its range ambiguity, for extended pseudo-random codes, is greater than pulse by the ratio T.sub.i .multidot.PRF. Its Doppler frequency ambiguity is 1/.tau., which in practical FFT filter processors is the same for conventional CW Doppler radars. Coherent processing is used for both CFMR and Doppler radars and the resulting gain is proportional to T.sub.i.
The range bin array of coherent signal time elements can be processed to improve range measurement accuracy and resolution. The time bandwidth characteristics of the transmitted wave are efficiently processed. For example, the total accumulated bandwidth can be utilized in deriving high range resolution. Thus, CFMR affords the same resolution capabilities as conventional pulse compressed radar. The period of the range frequency sequence is usually greater than the transit period to the most distant target of interest; but can be of any desired length consistent with scenario geometry and dynamics. This characteristic eliminates range ambiguities associated with high PRF pulse radars. Continuous operation provides a 100% duty cycle that minimizes peak power to an average value. It is the average value of power that determines radar performance. Non-ambiguous measurements of each target's range and velocity are simultaneously derived from the same waveform. These factors extend the capability of synthetic aperture applications in terms of operating range and ability to efficiently detect moving targets. This can eliminate the need for large aperture scanning antennas in radar systems operating from moving platforms. Furthermore, in fixed installations advantages can be accrued by utilizing small size, wide angle, transmitting antennas in combination with large aperture, multi-beam receiving arrays wherein parallel processing over the total period normally allotted to scanning a frame provides system gain in excess of the system loss introduced by the wider angular coverage of the transmitting antenna. Through parallel processing the data rate is no longer constrained by scanning requirements, but relates only to scenario dynamics. A large number of circuit elements are required to instrument parallel processes. In CFMR the circuits are identical and repetitive. Available signal processing resources can be used for wide area, coarse resolution surveillance; sector or restricted area, medium resolution search or zooming operations for high resolution target classification.
Conventional radar technology relative to such functions as sensitivity time control and moving target indications can be implemented. Advantages of frequency agility, spread spectrum and frequency related functions are inherent to the process. The instantaneous bandwidth of each frequency segment is less than that associated with conventional pulse radars. Peak powers are reduced to average values and processing losses associated with high duty cycle, staggered PRF radars are eliminated, thereby minimizing average power requirements. The segmented signal format allows maximum use of digital computer technology in terms of time, frequency, power management, adaptive control and signal processing. These factors are of prime importance when considering requirements for covert operation, jamming resistance and other problems that relate to maintaining operational capabilities in hostile environments. Improved resolution in the time, frequency and space domains is the solution to problems related to clutter, target classification, etc.